1. Field of the Invention
The present invention relates to an exhaust system for an internal combustion engine, and more particularly to a secondary air supply system for an engine exhaust gas purifying system. More particularly, the present invention pertains to a secondary air supply system wherein the air is drawn to the engine exhaust system under a suction pressure produced in the exhaust system.
2. Description of the Prior Art
In an engine exhaust system, there are pulsations of exhaust gas pressure wherein positive and negative pressures are alternately produced. Therefore, it is possible to draw the external air into the exhaust system by utilizing the negative pressure of the pulsations. In a secondary air supply system wherein the air is drawn under the suction pressure produced in the engine exhaust system, however, there is a possibility of exhaust gas blow back into the secondary air supply system under the positive exhaust gas pressure which produces unpleasant noises. In order to avoid such exhaust gas blow back and the noises caused thereby, a non-return valve may be provided in the passage of the secondary air. It has however been found that the non-return valve is not completely satisfactory in preventing the exhaust gas blow back and the noises caused thereby.
Japanese utility model application No. 48-117893 filed on Oct. 9, 1973 and disclosed for public inspection on June 4, 1975 under the disclosure number 50-60419 proposes a system which includes an expansion chamber provided in the secondary air passage upstream the non-return valve so that a noise attenuator is formed in the passage. In the proposed system, when combustible substances in the exhaust gas are burnt in the after-burning chamber and a high pressure combustion gas is produced, a part of the combustion gas tends to flow back into the secondary air passage. The non-return valve functions to block the reverse flow of the combustion gas, however, a small portion of the combustion gas may leak through the non-return valve. The portion of the combustion gas which has leaked through the non-return valve is then flowed into the expansion chamber where the gas is expanded. In this manner, the noise can be attenuated as in a conventional exhaust noise attenuator. It should however be noted that the proposed system is not completely satisfactory, either, in attenuating the noise to a satisfactory level. Further, since the expansion chamber of substantial volume is provided in the vicinity of the non-return valve, the flow of the secondary air to the exhaust passage is weakened so that a sufficient quantity of air cannot be introduced into the exhaust passage.
A desired quantity of air supply can be ensured by connecting the expansion chamber with the non-return valve through a narrow passage. However, this solution produces a further problem. It has been recognized that the noise caused by the blow back of the combustion gas into the secondary air passage generally includes low frequency components which are based on the exhaust gas noise transmitted through the non-return valve and medium and high frequency components which are produced at the non-return valve when the valve operates. The high frequency components can resonate with the secondary air passage between the expansion chamber and the non-return valve producing noticeable noises.
Japanese patent application No. 50-78130 filed on June 24, 1975 and published for public inspection on Jan. 7, 1977 under the disclosure number 52-1320 proposes to provide the secondary air supply system with an expansion chamber upstream the non-return valve for suppressing the low frequency noise components. The expansion chamber is connected with the non-return valve through a narrow secondary air passage which may be effective to ensure a sufficient supply of the secondary air. In order to prevent the medium and high frequency noise, there is provided a resonance chamber around the secondary air passage. It should however be noted that the proposed system is disadvantageous in that the system becomes bulky because of the expansion chamber and the resonance chamber. The resonance chamber may be substituted by extending the passage between the non-return valve and the expansion chamber to an appropriate length, however, this solution has a problem in that the system possesses an excessive overall length.